1. Field of the Invention
The field of the present invention is that of aeronautic propulsion, and in particular that of turboshaft engines with gas generator and free turbine.
2. Description of the Related Art
Turbine engines are commonly used for aircraft propulsion and in particular for propulsion and lift for rotary-wing aircraft or helicopters. These engines comprise a gas generator consisting of a compressor, an annular combustion chamber and a turbine known as a generator turbine which drives the compressor by means of a shaft, known as a generator shaft. The gas generator is generally a single spool gas generator, that is to say it comprises just one compressor and a single turbine, both connected by a single shaft, but it may also be a multi-spool gas generator, that is to say it may comprise a plurality of compressors and a plurality of turbines, each compressor being connected to a turbine by a specific shaft. The gases leaving the gas generator are then sent to a second turbine, known as a free turbine, which is associated with a power shaft, distinct from the shaft(s) of the generator, which provides the power needed for propulsion. This shaft drives a reduction gearbox connected to the main gearbox of the helicopter (or MGB), which drives the hub of the main rotor and the anti-torque rotor. This reduction assembly is generally integrated into an accessories gearbox for the equipment necessary for good operation of the engine or the aircraft.
For reasons of structural simplicity, the free turbine is generally positioned downstream of the last generator turbine and the shaft which it drives is coaxial with the shaft of the gas generator. This power shaft may exit towards the rear of the engine or alternatively, as is more usually the case, be concentric with the shaft of the gas generator and return towards the front. With the objective of compactness and/or ease of access, this configuration makes it possible to position the reduction gearbox and the accessories gearbox on a level with the air inlet of the gas generator.
These engines with concentric shafts, such as, for example that described in British Patent GB 594207, have the drawback of being complex to produce, which makes it somewhat difficult to produce them at relatively low cost. To this is added the complexity of the combustion chamber, due to its annular shape, which likewise hinders the reduction of production costs; this annular shape requires a large number of injection points, which, in small engines, complicates the introduction of devices for reducing nitrogen oxide (NOx) emissions.